None of the filming actually took place at sea. A four-fifths sized replica of the fishing schooner was built in the "tank". The actors merely walked on to it every day while filming. Distance and tracking shots of the schooner were a real ship that was filmed at sea and spliced into the movie where necessary.
This was one of the final films Lionel Barrymore made before his degenerative arthritis crippled him. The following year, he was hobbling around on crutches in Frank Capra's You Can't Take It with You (1938); after that, he was confined to a wheelchair for the rest of his life.
Spencer Tracy found the main challenge in playing Manuel was to put him across as a genuinely happy man, without making him seem like an idiot. MGM general manager Edward Mannix warned Tracy not to attempt an accent, because he would sound ridiculous.
Spencer Tracy was impressed early on by Freddie Bartholomew's dedication to the role, jumping over the side of the boat in order to get what he considered sufficiently wet after having been shot with a hose and doused with a bucket of water. "The kid can take it," Tracy said. "I hand it to him."
Spencer Tracy initially turned down the role of Manuel because he thought it was too secondary to the boy. He did not attempt a Portuguese accent but instead based his accent for the film on a Yiddish voice he had used during an early theater performance. He initially hated his performance in the movie until it won him good reviews and an Oscar nomination.
Spencer Tracy was initially reluctant to take on the part of Manuel, mainly because he had to sing in several scenes and get his hair curled. His new curly locks provided a lot of amusement to his friends and fellow actors. Joan Crawford, for instance, referred to him as "Harpo" (after Harpo Marx, the curly-haired Marx Brother).
In the novel (which first appeared as a serialization in "McClure's" magazine beginning November 1896) Harvey Cheyne is 15 and his father and mother travel by train from San Diego when they are notified Harvey has arrived in Gloucester. In the film his father says (at around 05 mins) "I wish his mother had lived to see him now, ten years old and yet he's one of the editors of his school paper."
The Canadian Fishing and racing schooner Bluenose was used for full view shots of the ship during the filming process. The Bluenose was a Canadian national icon, hailing from Lunenburg, Nova Scotia. She symbolized the pride of the Canadian fishing fleet, and was the winner of the International Fisherman's Trophy for 17 years.
A fishing schooner was bought by MGM in Gloucester, Massachusetts and sailed to Newfoundland for background shots. Then it went through the Panama Canal to California where Long Beach Harbor stood in for Gloucester.
The trivia item below may give away important plot points.
The Fishermen's Memorial at the end of the film is a replica of the one in Gloucester, Massachusetts. The actual memorial can be seen at the beginning of The Perfect Storm (2000), another movie about Gloucester fishermen.